It has been estimated by various studies that 70% of women fake orgasms at some point in their sexual lives. Some fake orgasms very regularly -- as in every time -- while others only fake it some of the time. The psychology behind faking orgasms is simple: She isn't going to have an orgasm this time, and she knows it. She can't be bothered with a) you trying hard to satisfy her and prolonging the encounter, and b) you feeling bad because you couldn't satisfy her come hell or high water. If you want to know if you are being duped, use the following signs that she really is having an orgasm to distinguish the faux from the bona fide, and catch her in the act.
Retraction of the clitoral head
This occurs just before orgasm and provides you with a clitoris-sized hint. When the clit disappears, you're on the right track, so don't stop. This coveted disappearance of the clitoris isn't only visible if you have the lights on and your face all up in it; it's something you can feel as well. So, get yourselves into a position where you can easily rub her clitoris during sex, and use it to your advantage. Under the guise of giving her some extra special treatment, you can feel whether she's actually close to climaxing or just faking.